This antireflection coating is a coating whose index letter n satisfies the equation n.sup.2 =n.sub.0 where n.sub.0 is the index of the substrate on which said coating is deposited and whose thickness is equal to .lambda./4n where .lambda. is the wavelength.
For a given material, the thickness may be adjusted so as to obtain a good antireflection coating. A "good" antireflection coating is one whose reflectivity is less than 10.sup.-3. This extremely low value imposes severe stresses on the thickness which needs to be adjusted to better than 10.sup.-2.
Thus, it is relatively difficult to obtain good antireflection coatings.
There exists one method to control the thickness of such a coating when the latter is deposited on a semi-conductive laser. During the depositing operation on one of the transmitting faces of a semi-conductive laser, this laser is fed with a constant current situated above the oscillation threshold of the laser. The light emitted by the latter through the rear face (opposite the one being processed) is measured by a photoreceptor. Its intensity depends on the reflectivity of the front face: the less this face is reflecting, the more losses occur in the resonator and the less intense is the light transmitted by the other face. Accordingly, the measured power reduces along with the reflectivity of the antireflection coating. By detecting the passage of this power through a minimum, it is possible to determine the moment when the antireflection coating presents the reflectivity minimum.
This method is described in the article by M. SERENYI and al and entitled "Directly controlled antireflection coatings for semiconductor lasers" published in the journal "Applied Optics", Mar. 1, 1987, vol. 26, No 5, pp 845-849.
This article describes another technique consisting of exciting the laser below the threshold (and no longer above) and of measuring the intensity of the spontaneous light transmitted through the face being processed (and no longer through the opposing face). In this case, the object here is to find a luminous power maximum, which is obtained when the reflectivity of the coating is at its minimum. This method (A) is less accurate than the previous method (B), as the spontaneous transmission spectrum is wider than the spectrum of the stimulated transmission obtained above the threshold.
These techniques (both A and B) have a large number of drawbacks: first of all, the photoreceptor used needs to be placed in such a way as to collect the light maximum; now, this requirement is incompatible with the need to avoid masking the face to be processed. Then, as this photoreceptor is sensitive to the light emitted in the depositing chamber (evaporation gun or plasma) to mask this stray light, it is necessary to use interferometric filters, which reduce the luminous intensity to be measured; this intensity reduction is all that much more hindering when at its minimum the intensity to be measured is already extremely low. Finally, a strict alignment needs to be effected between the laser and the photoreceptor with the aid of lenses, mirrors, optical fibers, etc., which significantly complicates the installation.